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At some level, everything in the universe is orbiting something else, often something many times larger. This is a function of gravity, as larger objects act against smaller objects, eventually drawing them in closer over time. Alternately, not all planetary or stellar bodies have a rotation, though all known planets in our solar system do. It is this rotation which provides day and night cycles and prevents one side of a planet's surface from being over-exposed to the damaging rays of the sun. It is theorized that Earth's rotation is the result of a massive planetary impact, which also gave Earth its iron core, spawned its moon and caused the 23.5 degree tilt of the planet's axis. As a planet or other body orbits its parent star, it does so in an oblong, or elliptical path. This means that instead of going around in a perfect circle, the orbit itself is elongated at either end. As a result, the Earth drifts from far away to closer to the sun throughout its orbital path.